As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferable to use amorphous silicon, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferable to use polycrystalline silicon, which can form a transistor having a high field-effect mobility. As a method for forming polycrystalline silicon, a method of performing high-temperature heat treatment or laser light treatment on amorphous silicon has been known.
In recent years, an oxide semiconductor has attracted attention. For example, a transistor which includes an amorphous oxide semiconductor containing indium, gallium, and zinc is disclosed (see Patent Document 1).
An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a channel formation region of a transistor in a large display device. A transistor including an oxide semiconductor has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
A transistor including an oxide semiconductor is known to have an extremely low leakage current in an off state. For example, a low-power-consumption CPU utilizing the low leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 2).
It is also disclosed that a transistor having a high field-effect mobility can be obtained by a well potential formed using an active layer including a semiconductor (see Patent Document 3).